In general, in controlling the amount of fuel injection of an internal combustion engine with an electronic fuel injecting device of the speed density type, the base amount of the fuel injection is determined so as to realize an approximate stoichiometrical air fuel mixing ratio, which base amount is obtained by effecting the rotational speed correction to the amount of the fuel injection determined by the two dimensional map of the rotational speed and the air intake pressure or determined in accordance with the air intake pressure. The thus determined base amount of the fuel injection is corrected by the coolant temperature, the intake air temperature, the battery voltage, and the like, and the corrected base amount of the fuel injection is used for control. The base amount of the fuel injection is determined mostly by the air intake pressure. The influence of the engine rotational speed on the base amount of the fuel injection is less than that of the air intake pressure.
In the above described internal combustion engine with an electronic fuel injecting device of the speed density type, when a disturbance is applied to the operation of the internal combustion engine which is running without load, both the rotational speed of the engine and the air intake pressure are caused to vary, and the amount of the fuel injection is caused to vary approximately in phase with the air intake pressure. Since the air intake system of the internal combustion engine with an electronic fuel injecting device of the speed density type includes a surge tank of a large volume, there occurs a phase difference between the variation of the rotational speed of the internal combustion engine and the variation of the air intake pipe pressure. Thus, a phase difference occurs between the variation of the rotational speed of the internal combustion engine and the variation of the amount of the fuel injection. Accordingly, as the rotational speed of the engine decreases, the air-fuel ratio becomes lean and the torque of the engine decreases, and accordingly the rotational speed of the engine further decreases. On the contrary, as the rotational speed of the engine increases, the air-fuel ratio becomes rich and the torque of the engine increases, and accordingly the rotational speed of the engine further increases. Therefore, variation of the rotational speed of the engine is amplified, whereby the rotational speed of the engine becomes unstable.
Under the above described situation, in the prior art internal combustion engine with an electronic fuel injecting device of the speed density type, the correction of the base amount of the fuel injection is carried out such that when the rotational speed of the engine increases over a predetermined rotational speed and the air intake pressure decreases below a predetermined pressure, the air-fuel ratio of the engine is made rich, while when the rotational speed of the engine decreases below a predetermined rotational speed and the air intake pressure increases over a predetermined pressure, the air-fuel ratio of the engine is made lean.
However, if a method for correction of the base amount of the fuel injection is adopted in which the air-fuel ratio is made rich when the air intake pressure decreases below a predetermined pressure, and is made lean when the air intake pressure increases over a predetermined pressure, it is not possible to expect the same control effect in all engines supplied by the manufacturer of the engines, because there are differences in the air intake pressures under the idle running condition of the supplied engines due to the slight differences that occur when engines are manufactured. Also, the air intake pressure under the idle running condition varies due to the friction of the engine, the long term variation of the air intake amount and the like, whereby the stability of the idle running and the exhaust gas emission tend to deteriorate. If an oxygen (O.sub.2) sensor feedback system using a three-way catalyzer for cleaning the exhaust gas is adopted in the car on which the internal combustion engine is mounted, the stability of the idle running is seriously deteriorated due to the air-fuel ratio variation caused by the feedback operation when the above described correction of the base amount of the fuel injection on the basis of the air intake pressure is used. Further, if the volume of the surge tank is made large in order to increase the output of the engine, the phase difference between the variation of the rotational speed of the engine and the variation of the fuel injection is caused to be enlarged and accordingly the rotational speed of the engine becomes unstable.